The present invention relates to an image inputting apparatus, such as a scanner or a digital copying machine, that reads document image information by scanning a document with a CCD line sensor.
Conventionally, a CCD line sensor used for a reduction optical system is configured by a photodiode array in which photodiodes serving as photoelectric conversion elements are arranged in a line, an analog shift register, a shift gate for transferring electric charges generated by the photodiode array to the analog shift register, a floating capacitor for converting the electric charges shifted in a main-scanning direction by the analog shift register into voltage signals, and an output amplifier for amplifying the voltage signals.
The sensitivity of a CCD line sensor which is defined as an output voltage with respect to an incident light amount per unit time varies because of sensitivity variation of a photodiode array and amplification factor variation of the output amplifier. Generally, variations of around ±20% of a reference value are set as the specification.
In the case of a 3-line color CCD sensor for reading a color document, color filters of three primary colors, namely, RED, GREEN and BLUE are placed on light receiving surfaces of three photodiode arrays disposed in parallel. Variation of transmittance of the color filters is superposed on the sensitivity variation. Thus, the sensitivity variation of the 3-line color CCD sensor is usually ±30%.
A cold cathode tube using a phosphor or a xenon lamp has been used as a light source in a scanner or a digital copying machine serving as an image inputting apparatus. The light emission amount of the light source is generally known to vary because of luminous efficiency variation due to variation of coated amount of a phosphor and degradation of the luminous efficiency of the phosphor over time.
As described above, performances of main parts of the image inputting apparatus vary widely. In order to suppress such variations, a number of control adjustments, and high precision assembly, are required.
As an example of control, light control is performed in order to control the light amount of a light source so that an output voltage from a CCD line sensor is constant. Generally, however, the higher the output a light source using phosphor is, the shorter its product life is. Further, such light sources have characteristics influenced greatly by ambient temperature. Accordingly, it is important to balance the life of a phosphor light source with the product life cycle of an image inputting apparatus as a system.
In accordance with the above-described method, it is considered that the signal to noise (S/N ratio) component included in an output signal from a CCD line sensor is not improved by the above-described control, but maintained. Consequently, methods for fundamentally suppressing sensitivity variation of a CCD line sensor and light amount variation of a light source have not been forthcoming.